Bifurcation can't create new lanes, only split the existing lane count up into separate logical slots.

What you're saying is possible though, you just need something a little heavier like a PCIe switch do the lane count + mixed-speed conversion magic. That's exactly what the chipset is, but for various reasons it's still only PCIe 4.0, even on the latest generation chips. I wouldn't be surprised if that changed again next generation. The downsides of this approach are switches add cost, latency, and can consume a lot of power. When they first upped the chipset to be a PCIe 4.0 connection in the 500 era, most motherboards capable of using the bandwidth of the chipset actually had to add chipset cooling.

Ideally they'd just add alternative options with more lanes directly from the CPU, but that'd add competition to the bottom of the Threadripper line.